⁴⁰Ar/³⁹Ar and K/Ar geochronology of the dykes from the South Indian granulite terrain

Abstract

⁴⁰Ar/³⁹Ar and conventional K/Ar data on mafic dykes and a felsic dyke, together with previous data have been used to constrain the ages of the dyke magmatism in the south Indian granulite terrain. The age of the mainly ENE-WSW-trending Agali-Anaikatti swarm is estimated to be 1980±25 Ma, with a very limited occurrence of upper Cretaceous (80-90 Ma) dykes in the area. The NW-SE Dharmapuri swarm is dated as at least 1800 Ma (possibly 1855±9 Ma). Both the NW-SE and the NE-SW dyke swarms of Tiruvannamalai are dated at 1650±10 Ma. Only a few dykes in north Kerala are of middle Proterozoic (ca. 1700-1650 Ma) age. Furthermore, the ⁴⁰Ar/³⁹Ar results of Dharmapuri dykes exhibit age spectrum characteristic of partial degassing; they provide strong evidence of latest Proterozoic overprinting by a thermal event probably related to the Pan-African orogeny. Most of the Phanerozoic dykes are confined to the coastal region in Kerala province. The dolerites of central Kerala and the majority of north Kerala dolerites intruded during the 70-65 Ma period; whereas a large gabbro and a felsic dyke in central and north Kerala, respectively, are of upper Cretaceous age (ca. 85 Ma) and the mean 81±1.5 Ma age from these dykes may represent cooling age. The oldest Phanerozoic (Upper Jurassic; 144±6 Ma) dyke intrusions have a restricted occurrence in south Kerala, very close to the southern tip of India. The Proterozoic dykes are interpreted as being lateral intrusions from under-plating igneous bodies beneath the eastern shield region. The geological record of the terrain does not show indications of significant pre-magmatic extension and rifting, but dyke magmatism may have led to extension, rifting and the consequent development of the India-Antarctica-Sri Lanka Proterozoic high-grade terrain. The data are also interpreted to suggest that the south Indian granulite terrain is an early Proterozoic tectono-metamorphic terrane with strong thermal imprints of Neoproterozoic/early Palaeozoic age. It is difficult to reconcile its extensions into Proterozoic Sri Lanka or Antarctica where tectono-metamorphism is dated to be of Neoproterozoic age. The late Phanerozoic dykes are predominantly related to the Deccan trap event, and are associated with mantle plume decompressional tectonics that gave rise to ocean floor spreading between the Seychelles and the Indian continent. The few older (ca. 145 Ma) dykes in south Kerala may be related to the rift tectonics that preceded the separation of Australia–Antarctica and India.